1. Field of the Invention
The invention relates to a voltage control circuit, and more particularly to a voltage control circuit of a variable gain amplifier circuit.
2. Description of the Related Art
A variable gain amplifier circuit generally comprises a multiplier, preferably a Gilbert cell, for amplifying an input signal. The variable gain amplifier circuit further comprises a voltage control circuit for controlling the multiplier, thus, the amplified input signal has the same polarity as an output signal. The voltage control circuit for a Gilbert cell typically comprises an input terminal and two output terminals. The voltage control circuit modulates the gain of the multiplier by control voltage generated by the two output terminals. During operation of the variable gain amplifier circuit, the control voltage generated by one of the two output terminals of the voltage control circuit must always be greater than that of the other output terminal. Thus, the polarity of the input signal of the multiplier is the same as the output signal, preventing abnormal operation.
FIG. 1(a) shows the output signal during normal operation of the variable gain amplifier circuit. One of the two control voltages generated by the voltage control circuit is always greater than the other control voltage, and the input and output signals of the multiplier have the same polarity, thus, the system output the normal output signal 11.
FIG. 1(b) shows the output signal when the variable gain amplifier circuit operates abnormally. The lower voltage exceeding the higher voltage suddenly at a time point T, results in the polarities of the input signal and the output signal are inverted at time point T, and an error signal is thus occurred. The system thus operates abnormally and outputs error output signal 12, complicating use of the variable gain amplifier circuit.
In a voltage control circuit, because a voltage source is immediately changed or the input voltage is not stable, the polarities of the voltage may run abnormally to make the whole circuit outputting wrong signal. So how to make the output voltage stable is a critical issue.